System for determining the position of a transponder

ABSTRACT

A system for determining the position of a transponder transmitting a signal and moving along a route is disclosed. The route includes a measuring station with at least two measuring points having on each side of the route and along a line generally perpendicular to the route. The measuring station includes a first receiver that receives the signal at one measuring point and a second receiver that receives the signal at the other measuring point. Each receiver provides an output. A circuit measures the phase difference between the outputs of the first and second receivers, and the transponder location along the line between the measuring points is determined based on the measured phase difference.

STATE OF THE ART

Such systems are known from the state of the art. In these systems ingeneral the object is to determine the position in the direction ofmovement whereby field strength measurements are used. An examplethereof is described in U.S. Pat. No. 5,621,411.

In certain cases it is desirable to know the position of the transponderin a direction transverse to the course. An example thereof are thetoll-installations on multi lane auto-routes. Therewith it is importantto make clear in which lane a vehicle is present before the necessarydata are exchanged with said vehicle in relation to the toll charging.Charging toll from a vehicle in a neighbouring lane has to be prevented.

An example of means for determining in which lane a vehicle is presentin the neighbourhood of a toll charging installation is described inU.S. Pat. No. 5,406,275. In this known system one detection station perlane is used whereby care has to be taken that each detection stationalmost exclusively detects its own lane and causes as less as possibledisturbance in the neighbouring detection stations. In this publicationalso the above mentioned distance measurements by means of fieldstrength measurements are described.

This prior art system is based on clearly distinguishable lanes and hasper lane separate hardware necessary to perform the requiredmeasurements.

Another example of circumstances whereby it is often desirable to knowthe position of the transponder in a direction transverse to the coursealong which the transponder is moving is formed by auto races, raceswith karats, skelters, bicycles of other vehicles, horseraces,houndraces and all other races which take place on a specific course.Especially at those places which are not in view of an observer it mightbe important to know which contestant has the innerlane, the outerlaneor moves on the middle of the road, etc.

OBJECT OF THE INVENTION

The object of the invention is now to determine the position of thetransponder in transversal direction in relation to a course without thenecessity to divide the course in transversal direction in clearlydistinghuised and electromagnetically screened lanes which each shouldhave its own measuring station.

BRIEF DESCRIPTION OF THE INVENTION

The above mentioned object is fulfilled by a system for determining theposition of a transponder, which transmits a signal and moves along aroute with at least a measuring station comprising antenna means forreceiving said signal at least at two measuring points positioned at thetwo outer points of a line segment which crosses the course in aperpendicular manner, whereby said measuring station comprises:

-   -   a first receiver for receiving said signal through said antenna        means at the one measuring point and    -   a second receiver for receiving said signal through said antenna        means at the other measuring point,    -   high frequency phase measuring means measuring the phase        difference between the output signal of the first receiver and        the outputsignal of the second receiver,    -   evaluation means which, based on the measured phase difference,        determines where the transponder passes said line segment.

I case the transponder is moving exactly in the middle of the road thenat both measuring points signal with equal phase will be received. Ifthe transponder is present more to the left side of the course then apredetermined phase difference will be measured. If the transponder ismoving more to the right side of the course then a predeterminedopposite phase difference will be measured.

Depending on the applied frequency and the width of the course it ispossible that a number of phase zeros will be measured spread over thelength of the line segment between both measuring points. That makes itimpossible for the evaluation unit to determine the position in anunambiguous manner.

There are a number of possibilities to eliminate this unambiguity. Inthe first place one could think of lowering the signal frequency.However, in general the applied frequencies are bounded to variousnational and international agreements which in general prevent avariation of the signal frequency. However, applying a modulation ispossible whereby a relatively low modulation frequency can be selected.In that case not the signal itself but the modulation frequency is usedfor the phase measuring.

A system which is embodied according to this principle has thecharacteristic that the transponder transmits a modulated signal, thatthe first receiver is followed by a first demodulator for demodulatingthe received signal, that the second receiver is followed by a seconddemodulator for demodulating the received signal, and that low frequencyphase measuring means measure the phase difference between the outputsignal of the first demodulator and the output signal of the seconddemodulator. In this system not the phase of the carrier wave but thephase of the modulating signal is measured of which the frequency ismuch lower and by means of which a line segment with a larger length(and therefore a course with a larger width) can be coveredunambiguously without a number of zero phase measuring points.

A disadvantage of the above mentioned system may be that the accuracy ofthe location determination based on relatively low frequency modulationsignal is lower than in case the higher frequency signals would be used.To solve said problem, it is preferred to combine both embodiments suchthat the evaluation means use the output signal of the low frequencyphase measuring means for “coarse” position determination whereas theoutput signal of the high frequency phase measuring means is used for“fine” position determining.

In principle various types of modulation can be used, amplitudemodulation, frequency modulation, phase modulation, etc. A type ofmodulation which needs only very simple circuits to obtain a properlyfunctioning system is amplitude modulation whereby the modulation signalis a pulse series by means of which the amplitude of the carrier wave ismodulated between 0% and 100%. In other words the transponder transmitssignal trains.

Another possibility to remove the uncertainty as resulting from variouszero crossings is reducing the line segment and apply a serious circuitof a number of smaller line segments. The length of each smaller linesegment has to be such that within each line segment an unambiguousmeasurement can be performed. To be able to determine which line segmentwill supply the correct measuring value use can be made of a fieldstrength measurement in each of the measuring points. The line segmentwhich is bounded by those measuring points which together have thestrongest sum signal is selected.

A system functioning according to this principle has the characteristicthat between both ends of said line segment another N measuring pointsare realised such that the line segment is divided by N+2 measuringpoints into N+1 segments each having a length which is small enough torealise an unambiguous measurement within said segment, whereby the N+2measuring points are connected to N+2 receivers, the output of each ofsaid receivers is connected to a field strength measuring means, theoutput signals of all field strength measuring means are evaluated in acomparison circuit, which comparison circuit transfers the outputsignals of those two receivers having together the largest fieldstrength, to a phase comparator to be mutually compared whereafter theresulting output signal of the phase comparator controls an evaluationunit.

Instead of field strength measurements a combination of carriermeasurements and modulation signal measurements is conceivable. In thatcase the system comprises an first elongated loop antenna which is usedfor p-hase measurements of the modulation signals at the end points inthe above described manner. The result thereof is a position with arelative low accuracy. The system comprises in that case a secondantenna having a series circuit of small loop antennas which are usedeach for a phase measurement based on the carrier signal at the ends ofeach small loop antenna. The position with low accuracy is used toselect one of the small loup antennas. The phase measurement on thisselected small loop antenna results into a position with a relativelyhigh accuracy. A disadvantage of this embodiment is the rathercomplicated antenna system, necessary for performing the measurements.

A further preferred embodiment of the system has according to theinvention the characteristic that the measurement is repeated a numberof times in a row, whereafter the results are interpolated such thatfrom the results the track can be derived which was followed by thetransponder within said course.

INDICATION OF THE FIGURES

The invention will be explained in more detail hereinafter withreference to the attached drawings.

FIG. 1 illustrates schematically a perspective view on a part of acourse, hereby at both sides of the course a receiving antenna of ameasuring station is installed.

FIG. 2 illustrates a top view on a measuring station with a loop shapedantenna on or in the surface of the course.

FIG. 3 illustrates another embodiment of the electronics in themeasuring station.

FIG. 4 illustrates an embodiment whereby the modulation signal is usedfor “course” position determination and the high frequency carriersignal is used for “fine” position determination.

FIG. 5 illustrates schematically an embodiment in which use is made ofan antenna consisting of the series circuit of a number of loops.

FIG. 6 illustrates schematically the exact route of a vehicle asfunction of a number of measurements performed by the system.

FIGURE DESCRIPTION

FIG. 1 illustrates schematically a part of a course 10, e.g. part of aroad, along which a transponder 12 is moving in the direction of thearrow 14. The transponder 12 will in a practical case be attached in oron an automobile, a motorbike or another vehicle, or to a human oranimal, and will thereby be moved along the course 10 in the indicateddirection.

At a number of places along the course measuring means are installed bymeans of which the position of the transponder 12 in transversaldirection can be determined. In FIG. 1 such a measuring post isillustrated comprising an antenna 16 at one side of the road and anantenna 18 at the other side of the road, an electronics unit 20 whichthrough a line 22 is connected to the antenna 16 and through a line 24is connected to the antenna 18.

During operation the transponder 12 will transmit with shortintermediate distances a signal which could be a continuous sinewavewith predetermined frequency but could also be a modulated carrier wave.Preferably in the last mentioned case the carrier wave is modulated by apulse series of significant lower frequency so that “signal trains” areformed.

For the coming part of this description it is assumed that thetransponder transmits a continuous and preferably sinusoidal signal.This transmitted signal is received by both antennas 16 and 18. Thereceived signals are transferred through lines 22 and 24 to theelectronics units 20 in which the signals are phase compared with eachother. If it is assumed furthermore that the signal lines 22 and 24 havethe same length then it will be clear that, in case the transponder 12is on the middle of the road, and the distance between the transponder12 and the antenna 16 is identical to the distance between thetransponder 12 and the antenna 18, both received signals in theelectronics circuit 20 have the same phase. A phase difference 0indicates therefore that the transponder 12 is in the middle of the road(or at least can be there). In case the transponder 12 is deviating fromthe middle of the road to the left then between both received signals acertain phase difference will be developed. If the transponder 12deviates from the middle of the road into the right direction then inboth received signals an opposite phase difference will be developed. Ifboth lines 22 and 24 are not exactly of the same length then this willcause a fixed phase difference for which compensation can be provided aswill be clear for the expert in this field. A similar note can be madeby other embodiments of the system which will be described hereinafter.

A disadvantage of the schematically illustrated system in FIG. 1 is thatthis system can be realised in practice only for rather high carrierfrequencies. Only then the dimensions of the antennas 16 and 18 will besuch they are allowable in practice. Many of the momentarily usedtransponder applications, for instance for tracking vehicles alongcertain road sections, make use of much lower carrier frequencies. Inthat case it is preferred to use another antenna configuration asschematically illustrated in FIG. 2.

In FIG. 2 the course in top view is indicated in general by 30. In animaginary coordinate system the direction of movement 14 of thetransponder 12 equals the Y-direction. Transversal to this direction, inother words in the width direction of the course 30 the X-direction isassumed whereby in the example of FIG. 2 the lower side of the courseoverlaps X=0 whereas the upper side edge of the course overlaps X=B,whereby B is the width of the course 30. On the traject a loop shapedantenna 32 is installed comprising two long parallel conductorsextending at short distance of each other which at X=0 and X=B areconnected by short transversal conductors. The short transversalconductors are through the conduits 34 and 36 in connection with theelectronic unit 38. In this electronic unit 38 two receivers 40 and 42are positioned as well as a phase measuring unit 44 and an evaluationunit 46.

The signals measured at the ends of the loop antenna 32 are throughlines 34 and 36 supplied to the receivers 40 and 42 and there amplifiedup to a desired level. The output signals of the receivers 40 and 42 arein a phase measuring unit 44 compared in phase with each other resultinginto a phase output signal. This phase output signal is supplied to anevaluation unit 46 which derives an X-value from this phase signal. Ifthe transponder is located exactly in the middle of the road then theunit 46 will provide a value X=B if the transponder is located more tothe lower side of the road then the unit 46 will for instance providethe signal X=X1, whereby X1<½B, whereas if the transponder 12 is morelocated to the upperside the unit 46 can supply for instance a signalX=X2 whereby X2>½B is.

Dependent on the selected carrier wave frequency and dependent on thewidth B of the course it will happen that a number of 0 points aredeveloping on the loup shaped antenna 32 so that the measurement is notunambiguous anymore. To provide a remedy it is for instance possible tomake use of a modulated carrier wave instead of a continuous carrierwave whereby for the phase measurement not the carrier wave but themodulation signal with a much lower frequency is used. The electronicsunit 38 a is in that case extended by 2 demodulators in the way, asschematically is illustrated in FIG. 3. A first demodulator 48 isinstalled between the receiver 40 and the phase measuring unit 44whereas a second demodulator 50 can be placed between the receiver 42and the phase measuring unit 44. By adding these both demodulators 48and 50 in the phase measuring unit 54 the phase difference between themodulation signals is measured. Because thereby signals with a very lowfrequency are involved it is now possible by a suitable selection of thefrequency to reduce the number of zero points in the output signal ofthe unit 44 to only one. The evaluation unit 46 is able therewith toindicate unambiguously within the course X=0 and X=B where thetransponder 12 is located.

A disadvantage of the use of relatively low frequency modulation signalscan be that the therewith-obtained accuracy in the positiondetermination is lower than in case the higher frequency carrier wave isapplied. In the embodiment which is schematically illustrated in FIG. 4the advantages of both embodiments are combined. The elongated loopantenna which is present in or on the course is in that case indicatedby 70. The ends of the antenna 70 are through lines 72 and 74 connectedwith the respective receiver 76 and 78. Each of the receivers supplies ahigh frequency modulated signal to one of the respective demodulators 80and 82. The lower frequent modulation signals at the outputs of thedemodulators 80 and 82 are supplied to the inputs of the phasecomparator 84.

The high frequency output signals of the receivers can be compareddirectly with each other in the phase comparator 86. As indicated abovethis may lead to a non-unambiguous location determination. By combiningthe output signal of the phase comparator 84, by means of which theposition is “coarse” indicated however not unambiguously, with theoutput signal of the phase comparator 86 it will be clear that withinthe “coarse” determination a “fine” tuning can be applied. Theevaluation unit 88 therefore provides as a result a locationdetermination with high accuracy.

Above one has assumed that the usual antenna is present in or on thesurface of the road. That is however not necessary. The transponder canbe embodied also as a vertical standing loop or window antenna. Also anantenna at a certain height above the road such, that the transpondercan move underneath the antenna, can be applied.

Another possibility to eliminate the ambiguity in the outputsignal ofthe phase measuring unit 44 is illustrated in FIG. 5. Instead of anelongated loop shaped antenna 32 such as in FIG. 2 or FIG. 4 in thiscase use is made of a series circuit of a number of much shorter loopantennas 52 a, 52 b, 52 c, . . . . Each of these antennas is through anown line 54 a, 54 b, 54 c . . . connected to an own receiver 56 a, 56 b,56 c, . . . in the electronic unit 38 c. The outputs of the variousreceivers are connected to a series of field strength meters 58 a, 58 b,58 c, . . . which supply output signals to a comparison circuit 60. Allthese output signals together form a curve which indicates where, abovewhich small antenna 52 a, 52 b, 52 c, . . . the transponder has to befound. The comparison circuit 60 in fact establishes which two adjacentreceivers have the largest summing amplitude of the received signals andcontrols the series of switches of 62 a, 62 b, 62 c, . . . such thatonly the output signals of these two selected receivers are transferredto the phase measuring circuit 64. The outputsignal of the phasemeasurement circuit 64 is taken into account by the evaluation circuit66 together with the positions of the switches 62 a, 62 b, 62 c, . . . .

Above it is assumed that the transponder is an active transponder whichtransmits signal trains with regular intervals without being activatedthereto by an externally received signal. The invention however can beapplied wit good results in combination with passive transponders whichbecome only active after reception of an activated signal and willtransmit then a response signal.

Finally it is remarked that in the above description a line segment isassumed which is perpendicular to the track direction and which has itsendpoints at the edges of the track. A line segment which is not ideallyperpendicular but makes a small angle with the direction of the tracksuch as caused by not accurately setting of the line segment can appeareasily in practice and will in general not lead to a grave measuringerror. Only if the angle is relatively large (larger than 10 degrees)then this angle should be taken into account in the evaluating means.

1. System for determining a position of a transponder, which transmits asignal and moves along a route with at least a measuring stationcomprising antenna means for receiving said signal at least at twomeasuring points positioned at the two outer points of a line segmentwhich crosses the course in a perpendicular manner, wherein saidmeasuring station comprises: a first receiver for receiving said signalthrough said antenna means at the one measuring point, a second receiverfor receiving said signal through said antenna means at the othermeasuring point, high frequency phase measuring means measuring thephase difference between the output signal of the first receiver and theoutput signal of the second receiver, evaluation means which, based onthe measured phase difference, determines where the transponder passessaid line segment.
 2. System according to claim 1, wherein the systemcomprises an elongated loop antenna comprising two parallel antennaconductors extending a short mutual distance and having a length equalto the length of said segment, which antenna conductors are connected attheir ends where the measuring points are formed.
 3. System according toclaim 1, wherein the system is adapted such that the measurement isrepeated a number of times in a row, whereafter the results areinterpolated such that from the results the track can be derived whichwas followed by the transponder within said coarse course.
 4. Systemaccording to claim 1, wherein the transponder transmits a modulatedsignal, that the first receiver is followed by a first demodulator fordemodulating the received signal, that the second receiver is followedby a second demodulator for demodulating the received signal, andwherein the system further comprises a second phase measuring unitadapted to operate at a frequency lower than the first-mentioned phasemeasuring means to measure the phase difference between the outputsignal of the first demodulator and the output signal of the seconddemodulator.
 5. System according to claim 4, wherein the evaluationmeans use the output signal of the low frequency phase measuring meansfor coarse position determination whereas the output signal of the highfrequency phase measuring means is used for fine position determination.6. System according to claim 4, wherein the modulated signal is obtainedby amplitude modulation whereby the modulation signal is a pulse seriesby means of which the amplitude of the carrier wave is modulated between0% and 100%.
 7. System according to claim 1, wherein that between bothends of said line segment another N measuring points are realized suchthat the line segment is divided by N+2 measuring points into N+1segments each having a length which is small enough to realize anunambiguous measurement within said segment, whereby the N+2 measuringpoints are connected to N+2 receivers, the output of each of saidreceivers is connected to a field strength measuring means, the outputsignals of all field strength measuring means are evaluated in acomparison circuit, which comparison circuit transfers the outputsignals of those two receivers having together the largest fieldstrength, to a phase comparator to be mutually compared whereafter theresulting output signal of the phase comparator controls an evaluationunit.
 8. System according to claim 7, wherein the antenna is built as aseries circuit of N+1 small loop antennas each comprising two parallelantenna conductors extending at short mutual distance of which the endsare interconnected, which loop antennas are in length direction coupledto each other.
 9. A system for determining a position of a transponder,which transmits a signal and moves along a route, the system comprising:an antenna assembly having a first point of measurement and a secondpoint of measurement, the points of measurement configured to define aline segment which crosses the course in a perpendicular manner, theantenna assembly adapted to receive said signal at each point ofmeasurement; a first receiver coupled to the antenna assembly andadapted to provide a first output signal based on said signal asreceived at the first point of measurement; a second receiver coupled tothe antenna assembly and adapted to provide a second output signal basedon said signal as received at the second point of measurement; a phasemeasuring unit coupled to the first receiver and the second receiver,the phase measuring unit adapted to provide a phase output signal basedon a phase difference between the first output signal of the firstreceiver and the second output signal of the second receiver; and anevaluation unit coupled to the phase measuring unit and adapted todetermine a position of the transponder along the line segment based onthe phase output signal.
 10. The system according to claim 9, whereinthe antenna assembly has N points of measurement between both ends ofsaid line segment such that the line segment is divided by N+2 points ofmeasurement into N+1 segments each having a length which is small enoughto realize an unambiguous measurement within said segment, and whereinthe system further comprises: N+2 receivers in total, wherein a receiveris coupled to a point of measurement and adapted to provide acorresponding output signal based on said signal as received at eachpoint of measurement; a field strength measuring assembly adapted toreceive each of the output signals and provide a corresponding outputsignal of field strength; and a comparison circuit adapted to receivethe output signals of field strength and adapted to determine the outputsignals of those two receivers having together the largest fieldstrength and adapted to provide said output signals to the phasemeasuring unit.
 11. The system according to claim 9, wherein the antennaassembly comprises an elongated loop antenna comprising two parallelantenna conductors extending a short mutual distance and having a lengthequal to the length of said segment, which antenna conductors areconnected at their ends where the first and second points of measurementare formed.
 12. The system according to claim 9, wherein the transponderis adapted to transmit a modulated signal by amplitude modulationwhereby the modulated signal is a pulse series where the amplitude ofthe carrier wave is modulated between 0% and 100%, and wherein thesystem further comprises: a first demodulator coupled to the firstreceiver and adapted to demodulate the first output signal; a seconddemodulator coupled to the second receiver and adapted to demodulate thesecond output signal; and a second frequency phase measuring unitcoupled to the first and second demodulators and adapted to measure aphase difference between an output signal of the first demodulator andan output signal of the second demodulator and adapted to operate at afrequency lower than the first-mentioned phase measuring unit.
 13. Thesystem according to claim 12, wherein the evaluation unit is adapted touse the output signal of the second phase measuring unit to determine acoarse position and wherein the evaluation unit is adapted to use theoutput signal of the first-mentioned phase measuring unit to determine afine position.
 14. A method for determining a position of a transponder,which transmits a signal and moves along a route, the method comprising:providing a first signal based on said signal as received at a firstpoint of measurement; providing a second signal based on said signal asreceived at a second point of measurement, the second point ofmeasurement being positioned relative to the first point of measurementto define a line segment which crosses the course in a perpendicularmanner; measuring a phase difference between the first signal and thesecond signal; and determining a position of the transponder along theline segment based on the measured phase difference of the first andsecond signals.
 15. The method according to claim 14 and furthercomprising: providing an antenna assembly having N points of measurementbetween both ends of said line segment such that the line segment isdivided by N+2 points of measurement into N+1 segments each having alength which is small enough to realize an unambiguous measurementwithin said segment; wherein providing the first signal based on saidsignal as received at the first point of measurement and providing thesecond signal based on said signal as received at the second point ofmeasurement comprises providing N+2 signals in total based on saidsignal as received at each of the N+2 points of measurement; measuring afield strength at each of the N+2 points of measurement; and determiningthose two signals having together the largest field strength; andwherein determining the position of the transponder along the linesegment comprises using those two signals having together the largestfield strength.
 16. The method according to claim 14 and furthercomprising providing an antenna assembly including an elongated loopantenna having two parallel antenna conductors extending a short mutualdistance and having a length equal to the length of said segment, whichantenna conductors are connected at their ends where the first andsecond points of measurement are formed.
 17. The method according toclaim 14, wherein the transponder is adapted to transmit a modulatedsignal by amplitude modulation whereby the modulated signal is a pulseseries where the amplitude of the carrier wave is modulated between 0%and 100%, and wherein the method further comprises: demodulating thefirst signal; demodulating the second signal; and measuring a phasedifference between the demodulated first signal and the demodulatedsecond signal.
 18. The method according to claim 17, wherein determininga position comprises: determining a coarse position based on the phasedifference between the demodulated first signal and the demodulatedsecond signal; and determining a fine position based on the phasedifference between the first signal and the second signal.